Bi-directional amplifiers (BDA's), or repeaters, are commonly used for in-fill coverage or range extension in land mobile radio (LMR) networks. Typically, a BDA will receive a signal from a donor base station transmitter, boost the signal level, and retransmit the signal to mobile receivers lying within a designated area that would otherwise have weak or non-existent direct coverage from the donor base station transmitter. This sequence is commonly referred to as signal boosting in the “downlink” path. Conversely, a BDA will receive a signal from a mobile transmitter within the designated area, boost the signal level, and retransmit the signal back to the donor base station receiver (commonly referred to as the “uplink” path). Typical examples of the use of BDA's are to provide radio coverage into vehicular tunnels, underground car parks, shopping centres, or coverage extension along a motorway through a daisy chain of BDA's.
There are standards that govern the use of RF spectrum by all radio transmitting and receiving devices. These standards are established and mandated generally by government agencies that are chartered to ensure that the spectrum is allocated and used appropriately. Radio devices must conform to these standards in order to minimise both spectral pollution caused by spurious emissions, and interference to spectrum users.
The implication, as far as BDA's are concerned, are that the devices need to be channelized in order to meet relevant criteria within the standards. Simple broadband BDA's can only be employed in underground sites, where adverse spectral impact is mitigated by the ground's RF absorption, and so the usefulness of these devices is limited.
Channelizing is typically achieved by conversion of the input RF frequency to an intermediate frequency (IF) using standard mixing techniques, then filtering this IF signal at the wanted channel frequency (typically at 12.5 kHz or 25 kHz channel spacing), followed by conversion back to an RF frequency for subsequent re-transmission. Typically, the channel filters in this instance are analogue crystal or SAW filters, and the input and output RF frequencies are either identical (non-translating mode) or different (translating mode).
The technological advancements in semiconductor devices that have occurred over the past few years have led to high speed digital devices that can be used to process signals at frequencies of 100 MHz or more. Given that typical radio communication networks that operate on frequencies ranging from 60 MHz to 5.8 GHz for example, may be mixed to intermediate frequencies of say around 100 MHz, means that these devices may now be adapted for use in radio communication infrastructure products.
Digital signal processing devices that are now available, such as analogue to digital converters (ADC's), Field Programmable Gate Arrays (FPGA's) and digital to analogue converters (DAC's) are examples of the advancements that have been made. ADC's can now operate with clock speeds that enable direct analogue to digital conversion of RF signals at 100 MHz, say, and have 16 bit resolution which provides outstanding dynamic range. Equally, FPGA's and DAC's are available to complement the performance of the ADC's.
As a consequence, channelized BDA's for use in LMR networks, wherein the channelizing filters are implemented in the digital domain using well established filter implementation techniques, are now readily available. FIG. 1 is a typical example of one such configuration 100.
The arrangement in FIG. 1 shows a simplified block diagram 100 of an arrangement whereby a single digital signal processing (DSP) block 102 is configured as a downlink repeater to provide eight separate channelized outputs 104a-104h (wanted channels) from one input. While FIG. 1 includes elements embodying the present invention, as described in the following detailed description, the general architecture is similar to existing DSP-based repeaters. The input consists of a single ADC 106 fed from an IF stage 108. A second, identical arrangement (not shown), is provided to implement uplink (i.e. bi-directional) signal boosting.
These DSP-based BDA's (repeaters) will hereinafter be referred to as DSPbR's.
Typically, BDA's have input filters 110 to restrict the incoming RF signals to suit the band of interest. In general, these filters have a relatively large pass bandwidth (20 MHz, for example), therefore all of the input circuitry in BDA's, prior to the channelizing filters, will be subjected to a multitude of channel frequencies lying within the input filter bandwidth, across a range of input signal levels. In uplink, in particular, mobile transmitters may be close to the BDA, where input signal levels could well exceed −20 dBm, or they could be far away from the BDA where the input signal level could be as low as −110 dBm. This represents a minimum dynamic range of 90 dB that must be handled by the BDA.
BDA maximum power output levels may be of the order of +45 dBm, and maximum gain through the BDA may be of the order of 130 dB, meaning that as soon as input levels reach −85 dBm, gain control is required to keep the output level constant as the input level increases. This control mechanism is needed to ensure that the output stages are not overdriven beyond maximum rated output power, which would otherwise cause spurious emissions.
In DSPbR's, particular attention needs to be paid to ensuring that the ADC input does not exceed its rated full scale input voltage which would otherwise cause clipping and subsequent RF spurious emissions. Furthermore, the ADC could also be damaged by high input voltages. Similarly, input levels to the DAC's eg 112, need to be kept below that required to obtain rated full scale DAC output in order to avoid similar output spurii or device failure. The control of input signal levels, and therefore signal amplification (gain) at various stages within the DSPbR circuitry, is crucial for device protection and controlling spectral pollution.
This presents challenges, particularly in the uplink direction due to the dynamic range of the possible input signal levels, since any gain reduction or attenuation of high level input signals prior to the ADC input will have the effect of reducing the sensitivity of the DSPbR. That is, there exists the very real possibility that two wanted signals are present at the input of the DSPbR, one at a very low level that is just above the sensitivity threshold of the DSPbR, and one at a high enough level such that attenuation (net gain reduction) is required; remembering that this attenuation is common to all incoming signals the end result is that the signal level of the low level wanted signal is reduced as well thereby rendering it indiscernible from noise, and as a consequence, the signal is lost.
Clearly, the gain control circuitry must address all of these issues in such a way that spurious emissions are kept within regulatory limits, and that the sensitivity of the DSPbR is not unnecessarily impacted leading to compromises in performance.
The individual channel frequencies of the DSPbR are specified by user requirements. It is critical that the DSPbR only boosts the programmed channels, which in turn means that it must only respond to programmed channel frequencies, and that its boosted, filtered output must also fall directly on the programmed channel frequency. As mentioned previously, input and boosted output channel frequencies may be the same (non-translating mode), or different (translating mode), depending on user requirements.
Interference to other spectrum users will occur if the DSPbR does not faithfully respond to or output the exact programmed channel frequencies. That is, the frequency stability of the DSPbR is a critical specification and must be tightly controlled to minimize this interference.
The frequency stability of the DSPbR is determined by the stability of the frequencies of the local oscillators (LO's), eg 114, 116, that are used in the mixing processes to convert from RF to IF and then back to RF again, and by the stability of the numerically controlled oscillators (NCO's) in the digital processing circuitry which ultimately determine the centre frequency of the channelizing filters.
FIG. 1 shows a typical arrangement used to achieve this stability. The LO's, eg 114, 116 and NCO's (within DSP 102, not shown) are controlled by a reference generation subsystem 118 which numerically relates the LO and NCO frequencies to a known, stable reference frequency. A 10 MHz reference frequency is generated by a voltage-controlled, temperature-compensated crystal oscillator in the DSPbR for this purpose. This provides typical frequency stability of +/−1 ppm over a wide temperature range, which at 400 MHz for example, equates to +/−400 Hz. This has the effect of offsetting the channelizing filters by this amount which in turn has consequences in terms of unwanted spectrum emissions.
Better frequency stability over temperature variations can only be achieved by locking the 10 MHz reference frequency to an external, high stability master reference frequency. A perfect source of this master reference frequency is the highly accurate timing reference pulse extracted from the Global Positioning System (GPS) satellites via a GPS receiver 120. The reference generation subsystem in the DSPbR uses the GPS timing reference as the master reference frequency to further stabilize its internally generated 10 MHz reference frequency.
Under certain circumstances, the GPS master reference may not be available in which case the internally generated 10 MHz reference may drift giving rise to spectrum interference.
For example, in the event of a power failure, and subsequent restoration of power, the GPS receiver may take up to several minutes to acquire the satellites and establish a locked state.
In some cases, the DSPbR may be installed in a location where the visible sky aperture is limited (such as a valley) meaning that only a small number of satellites are in view at any point in time. Under these circumstances, the GPS receiver may temporarily lose satellites as they orbit and move out of the sky aperture, and re-acquire only as other satellites pass by.
Clearly, a method of maintaining the DSPbR's frequency stability in the absence of the GPS master reference is needed.